Polymer Synthesis CHEM 421 Conjugated Polymers & Applications.

Polymer SynthesisCHEM 421

Conjugated Polymers & Applications


Outline

• Conjugation

• Bandgap of conjugated polymers

• Synthesis

• Processing

• Applications: OLED, OFET, OPV


p Orbitals

p

atom diatomic molecule extended molecule

band

band

+_

……


bonds

electron density builds upbetween the nuclei

The wavefunctions cancel betweenthe nuclei.

Bonding combination of p orbitals ()

Antibonding combination of p orbitals (*)


Larger molecules

Atkins, Physical Chemistry

butadiene

0 nodes

1 node

2 nodes

3 nodes

To generate the molecular orbitals of larger molecules, we take linear combinations of the atomic orbitals from each atom. If there are n atoms, there will be n different combinations. With a littlebit of math, the energy levels and wavefunctions to the right can be found.

fully bonding

fullyantibonding

partiallybonding

partiallyantibonding

Top down viewof the p orbitals

Polymer SynthesisCHEM 421Benzene

Atkins, Physical Chemistry

The electrons

The electrons

Top down viewof the p orbitals


n

PA: polyacetylene(1st conducting polymer)

PPV: poly(phenylene-vinylene)(used in 1st polymer LED)

PT: polythiophene(widely used in transistors)

PPP: poly(para-phenylene)(large bandgap)

S

S

n

n

Conjugated polymers


Band diagram for poly(para-phenylene)

E.K. Miller et al., Phys. Rev. B, 60 (1999) p. 8028.

n

PPP

Benzenelevels

-/a 0 /a

k

Eg


Tuning the bandgap of conjugated polymers

~ 2 eV~ 3 eV


Tuning the bandgap of polythiophene derivatives

Mats Andersson et al. J. Mater. Chem., 9 (1999) p. 1933.

which one has the largest bandgap?


How to make conjugated polymers:1. precursor methods

The Durham precursor route to polyacetylene.

The Wessling-Zimmermann route to PPV.

water-soluble polyelectrolyte

Feast et al. Polymer 37 (1996) p. 5017.


How to make conjugated polymers:2. polycondensation

Commonly Used Coupling Reactions• Stille Coupling• Suzuki Coupling• Heck Reaction• Ullmann Reaction• Sonogashira Coupling• Kumada and Negishi Coupling

X YX Y [ ]n

Polymer SynthesisCHEM 421Stille Coupling

Pd(II)L2Cl2 + 2R’SnBu3

R’-R’ + 2Bu3SnClR-R’

Pd(0)L2

RX

RPdL2XRPdL2R’

R’SnBu3

Bu3SnX

3. Reductive

Elimination

1. Oxidative addition

2. Transmetalation


Suzuki Coupling Ullmann Reaction

Heck Reaction

http://www.organic-chemistry.org

Sonogashira Coupling


Kumada Coupling

Negishi Coupling


Different approaches to one polymer


n-type doping

Ene

rgy

Polymer

e-

Reducing agents donate electrons to the conduction band.

Solids like calcium, lithium and sodium tend to dope the polymer only near the surface since they cannot diffuse into the film. Electrolytes (see below) can be used to dope an entire film.

(-polymer)n + (Na+(Naphthalide)-]y -> [(Na+)y(-polymer)-y]n + y(Naphth)0

Lithium

Reducedpolymer

Counter-ionOxidized molecule


Polyethylene dioxythiophene polystyrene sulphonate (PEDOT/PSS) can bebought from Bayer as an aqueous solution under the trade name Baytron.

The sulphonic acid group on the PSS dopes the PEDOT to make it conductive.

PEDOT

PSS

p-type doping


The Nobel Prize in Chemistry 2000


Small conjugated molecules

anthracene

tetracene

pentacene

It is not impossible to solution depositsmall molecules, but it is usually hard to make high quality films because the solution viscosity is too low. Small molecules are usually deposited from the vapor phase.

Mobilities of >1 cm2/V-s can beachieved by thermally evaporating thin films. The mobility is limited by grain boundaries.

Mobilities > 3 cm2/Vs can be achieved by growing single crystals.


Thermal evaporation of small molecules

Substrate

Host Dopant

Heater

Pump

Shutter

The pressure is low enough for the meanfree path of a molecule to be larger than thesize of the chamber.

Impurities with a higher vapor pressure thanthe desired molecule are deposited on theshutter.

Peter Peumans


Depositing polymers: Spin coating

Procedure1. Dissolve the material.2. Cast the solution onto the substrate.3. Spin the substrate at 1000 to 6000 revolutions per minute.

Most (~ 99 %) of the solution is flung off of the substrate, but a high-quality thin film is left behind.

The thickness of the films goes up with increasing solution concentration and down with increasing spin speed. Polymers with larger molecular weights tend to result in more viscous solutions, which yield thicker films.

Polymer SynthesisCHEM 421Features of spin coating

Advantages• Spin coating can be done at atmospheric pressure and is very cheap.• Film thickness of up to several hundred nanometers can be obtained.• The thickness can be controlled (but not as well as with evaporation).• The thickness is fairly uniform across the substrate (except at the edge).

Disadvantages• The whole substrate is coated. Patterning must be done separately.• In most cases, part of the material must be nonconjugated so that the molecules are soluble.• In can be difficult to make multilayer structures because the deposition ofone layer can dissolve the layer underneath.


Screen printing

• This technique is used to put patterns on T-shirts.• The squeegee is used to press the dye through the screen. • Recently screen printing of polymers has been used to make LEDs and photovoltaic cells.

Ghassan Jabbour et al. Adv. Mater. 12 (2000) p. 1249.

This LED doesn’t have perfectly uniform emission, but it isn’t badfor the first demonstration of this deposition method.


Advantages and disadvantages of screen printing

Advantages

• Large areas can be covered at low cost.

• Atmospheric pressure.

• Patterning is possible.

Disadvantages

• Controlling film thickness might be difficult.

• The material must be soluble and have a viscosity within a certain range.


Drop casting

Hotplate

Petri DishSubstrate

Drying solution that leavesa film behind.

Covering the sample with a Petri dish slows down the evaporation rate,which results in more uniform films.

Keeping the substrate level results in much more uniform films.

Since the solvent evaporates slowly, the material can crystallize or aggregate intofairly well ordered structures. (This can be good or bad.)

Polymer SynthesisCHEM 421Summary of drop casting

Advantages• Film thickness can be far greater than 1 m.• This method is very inexpensive.• In many cases the film can be removed from the substrate by peeling or by dissolving the substrate (which would be NaCl or KBr).

Disadvantages• The film thickness is difficult to control and is not very uniform.• Very thin films are difficult to make.• The material must be soluble.

Polymer SynthesisCHEM 421Ink Jet Printing (IJP)

Polymers can be deposited from a printer onto a substrate.

Advantages

• Patterning with resolution approaching 5-10 m is possible.

• No material is wasted. (~ 99 % is wasted with spin casting)

• Cost can be extremely low

Disadvantages

• Controlling film thickness is difficult.

• Fabrication of multilayer structure is difficult (compared to evaporation)

Polymer Synthesis CHEM 421 Conjugated Polymers & Applications.

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